November 7, 2018

One of the most important features in Gradle is the support for incremental tasks. Incremental tasks have input and output properties that can be checked by Gradle. When the values of the properties haven't changed then the task can be marked as up to date by Gradle and it is not executed. This makes a build much faster. Input and output properties can be files, directories or plain object values. We can set a task input property with a date or date/time value to define when a task is up to date for a specific period. As long as the value of the input property hasn't changed (and of course also the other input and output property values) Gradle will not rerun task and mark it as up to date. This is useful for example if a long running task (e.g. large integration test suite) only needs to run once a day or another period.

In the following example Gradle build file we define a new task Broadcast that will get content from a remote URL and save it in a file. In our case we want to save the latest messages from SDKMAN!. If you don't know SKDMAN! you should check it out!. The Broadcast task has an incremental task output property, which is the output file of the task:

We can run the task downloadBroadcastLatest and the contents of the URL is saved in the output file. When we run the task a second time the task action is executed again and the contents of the URL is fetched again and saved in the output file.

Suppose we don't want to access the remote URL for each task invocation. One time every hour is enough to get the latest messages from SKDMAN!. Let's add a new incremental task input property with the value of the current hour to the task downloadBroadcastLatest.

// File: build.gradle
...
task downloadBroadcastLatest(type: Broadcast) {
// Add incremental input property, with value that changes only
// every hour. Gradle will mark the task as up-to-date for
// every invocation as long as the hour value hasn't changed.
// For example to set the value so that the task is only
// execute once a day we could use java.time.LocalDate.now().
inputs.property 'check_once_per_hour', java.time.LocalDateTime.now().hour
outputFile = file("${buildDir}/broadcast.latest.txt")
}
...

Another option in our example is to add the incremental task input property to the source of our Broadcast task. We can do that, because we have written the task class ourselves. If we cannot change the source of a task the previous example is the way to add an incremental task input property to an existing task. The following code sample adds an input property to our task definition:

Finally it is important that to make this work a task has to have at least an increment task output property. If an existing task doesn't have one, we can add a outputs.upToDateWhen { true } to a task configuration so Gradle recognises the task as being incremental with output and the output is always up to date. In the following example we create a new task Show without an incremental task output property. In the task showBroadcastLatest we define that the task has an always up to date output: